It has become both desirable and necessary to reduce the hazardous substance content of industrial flue gasses. The hazardous substances can have a deleterious affect on the public health and the environment. Industry and government have been working to reduce the emissions of such substances with good progress being made. Special focus has been on flue gas from coal-fired boilers, such as that found in electric generation plants. Recent focus has also been on emissions from cement kilns. But there is more to do. Hazardous substances include, particulates, e.g. fly ash, acid gases, e.g. SOx, NOx, dioxins, furans, heavy metals and the like.
The methods used to mitigate the emission of hazardous substances depend on the nature of the hazardous substance, the minimum emission level sought, the volume of emitted gas to be treated per unit time and the cost of the mitigating method. Some hazardous substances lend themselves to removal from gaseous effluent by mechanical means, e.g. capture and removal with electrostatic precipitators (ESP), fabric filters (FF) or wet scrubbers. Other substances do not lend themselves to direct mechanical removal.
Hazardous gaseous substances that are present in a gaseous effluent present interesting challenges, given that direct mechanical removal of any specific gaseous component from a gas stream is problematic. However, it is known, and an industrial practice, to remove hazardous gaseous components from a gaseous effluent by dispersing a fine particulate adsorbent evenly in the effluent to contact and capture, in flight, the targeted gaseous component. This is followed by mechanical removal of the adsorbent with its adsorbate from the effluent vapor by ESP, FF or wet scrubbers. A highly efficacious adsorbent is carbon, e.g., cellulosic-based carbons, powdered activated carbon (PAC), etc. Such PACs, for example, can be used with or without modification. Modified PACs may enhance capture of the target hazardous substance by enhancing adsorption efficiency. PAC modification is exemplified by U.S. Pat. No. 4,427,630; U.S. Pat. No. 5,179,058; U.S. Pat. No. 6,514,907; U.S. Pat. No. 6,953,494; US 2001/0002387; US 2006/0051270; and US 2007/0234902. Cellulosic-based carbons include, without limitation, carbons derived from woody materials, coconut shell materials, or other vegetative materials.
A problem with the use of cellulosic-based carbons in industrial applications, is their unreliable thermal stability, that is, the lack of assurance that they are resistant to self-ignition. Self-ignition is especially problematic when the cellulosic-based carbon is used in the treatment of warm or hot gaseous effluents or when packaged or collected in bulk amounts. For example, bulk PAC is encountered (i) when the PAC is packaged, such as in super-sacks or (ii) when formed as a filter cake in an FF unit or is collected in silos or hoppers associated with an ESP, TOXECON unit, and baghouse. Self-ignition results from unmitigated oxidation of the carbon and can lead to its smoldering or burning. Self-ignition is exacerbated by the carbon being warm or hot, as could be the case when used in treating coal-fired boiler effluents. If oxygen (air) is not denied to the oxidation site or if the site is not cooled, the heat from the initial oxidation will propagate until the carbon smolders or ignites. Such an ignition can be catastrophic. Utility plants are especially sensitive about self-ignition as smoldering or fire within the effluent line can cause plant shut-down with widespread consequences to served customers.
Further information on PAC thermal stability can be found in U.S. Pat. No. 6,843,831, “Process for the Purification of Flue Gas.” Some carbons are more resistant to self-ignition than others. For example, in the US, the use of coal-derived PACs is the industry standard for utility flue gas treatment, in part because of the good thermal stability of coal-derived PACs.
It would be advantageous if PACs of lesser thermal stability, such as, those derived from certain cellulosic-based carbons could be modified to be more thermally stable so that the practitioner could enjoy the benefit of the excellent adsorption qualities of cellulosic-based carbons.